Tantalum carbide-tantalum fiber composite material

ABSTRACT

The thermal stress resistance of refractory metal carbides is substantially improved by the addition of chopped metal fibers. A composite consisting of 50 vol percent TaC-50 vol percent chopped Ta wire shows approximately a 40 percent improvement in thermal stress resistance over that of pure TaC.

United States Patent [1 1 Riley et al.

[ TANTALUM CARBIDE-TANTALUM FIBER COMPOSITE MATERIAL [75] Inventors:Robert E. Riley; James M. Taub,

both of Los Alamos, N. Mex.

[73] Assignee: The United States of America as represented by the UnitedStates Atomic Energy Commission, Washington, DC.

22 Filed: Jan. 18,1972

211 Appl. No.: 218,686

[52] US. Cl. 29/l82.8, 75/204, 75/DIG. 1 [51] Int. Cl. C22c 29/00 [58]Field of Search 75/DIG. 1, 203, 204;

[56] References Cited UNlTED STATES PATENTS 11/1966 Jens 75/DIG. 1

[ Dec. 18, 1973 3,149,253 9/1964 Luebke 75/D1G. 1 3,479,155 11/1969 Rudy29/1827 3,114,197 12/1963 De Bois et al. 75/D1G. 1 3,352,650 11/1967Goldstein et al 75/D1G. 1

Primary Examiner-Carl D. Quarforth Assistant ExaminerE. A. MillerAtt0rneyJ0hn A. Horan [57] ABSTRACT 1 Claim, 1 Drawing Figure TANTALUMCARBIDE-TANTALUM FIBER COMPOSITE MATERIAL BACKGROUND OF THE INVENTIONThe invention described herein was made in the course of, or under, acontract with the U. S. ATOMIC ENERGY COMMISSION. It relates tostructural materials having improved thermal stress resistance attemperatures in excess of 2,000 C and more particularly to suchmaterials consisting of refractory metal carbide-refractory metal fibercomposites.

It is known in the art that structural materials exposed to temperaturesin excess of 2,500 C for any substantial period of time, i.e., for morethan 60 minutes, rapidly degrade. Refractory metal carbides are obviouscandidates for such structural materials. However, they are brittle andreadily crack in such temperature regimes. This cracking in turn rapidlyleads to complete fracturing which may result in a catastrophic failureof the particular structure. As used within this application, fracturingmeans the separation of a massive body into two or more smaller bodies.

SUMMARY OF THE INVENTION We have now found that the addition of achopped metal fiber to the refractory metal carbide will substantiallyenhance the thermal stress resistance of the carbide. The refractorymetal carbide-metal fiber composites may readily be formed through hotpressing. While various refractory metal carbides may be used as thematrix material, it is desirable that the carbide be a monorather than adicarbide. Various chopped metal fibers such as Ta, Mo, 75 vol percentW-25 vol percent Re, Re, etc., may be used. In particular, choppedrhenium fibers are useful because rhenium does not form a carbide. Thelimiting factor in the use of a particular metal fiber in the compositesof this invention is the formation of the metal carbide-carbon eutectic.It will be apparent that the use of a metal fiber M as a reinforcingmaterial in a metal carbide matrix is undesirable if the temperature ofthe MC-C eutectic is below that to which the composite will be exposedin its structural use. In the case of rhenium, the limiting temperatureregime is that at which the Re-C eutectic occurs. The temperatures atwhich the various refractory metal carbides form eutectics with carbonare well known in the art.

We have found that a composite consisting of 50 vol percent TaC-50 volpercent chopped Ta wire shows approximately a 40 percent improvement inthermal stress resistance over that of pure TaC. At temperatures inexcess of 2,500 C, a composite material of this type will still crack,but the presence of the fibers inhibits the propagation of the crackscompletely through the structure so that fracturing is avoided orsubstantially reduced. As can be seen from the FIGURE, up to about 70vol percent Ta, TaC-chopped Ta wire composite materials exhibitisotropic thermal characteristics. This indicates that neither the sizenor the diameter to length ratio of the chopped metal fibers is criticalin the composites of this invention. Indeed, it suggests that the metalneed not be present as fibers at all but may serve as well in anirregular particulate form.

BRIEF DESCRIPTION OF THE DRAWING The FIGURE compares the results ofsteady-state thermal stress testing of various TaC-chopped Ta wirespecimens machined in both the parallel and perpendicular to thepressing direction with that ofa pure TaC specimen tested in theparallel direction only.

DESCRIPTION OF THE PREFERRED EMBODIMENT Tantalum wire 20 mils indiameter was chopped to approximately 0.25 inch lengths and blended indesired proportions with TaC powder having 1.5 micron nominal size. Theblended material was then hot pressed at l,800 C for a time sufficientto produce a density of about to percent theoretical. Typically, thistook about onehalf hour.

Steady-state thermal stress specimens were machined from the 10, 50, 60,and 70 vol percent Ta wire billets in both the parallel andperpendicular to the pressing direction. The FIGURE compares the resultsof steadystate thermal stress testing on these materials with that ofpure TaC specimen tested in the parallel direction only. The percent TaCmaterial was 96 percent of theoretical density. There is clearly asignificant increase in thermal stress resistance associated withcertain contents of Ta fiber. With the exception of the 30 vol percentTaC-70 vol percent specimens, the material is isotropic. Whereas purerefractory metal carbides have steady-state thermal stress resistancesin the range of 500 to 650 C radiator temperature, the 50 vol percentTaC-50 vol percent chopped Ta wire composite material has a steadythermal stress resistance at about 900 C radiator temperature. This isan improvement of about 40 percent over that of TaC alone.

One-inch-o.d. by one-quarter-inch-i.d. by one sixteenth-inch-thickspecimens of TaC reinforced with 60 and 70 vol percent chopped Ta wirewere thermally shock tested. There was a large difference in the powersetting required to initiate a crack and the power setting required tocompletely fracture the materials. Typ ically, a crack could be made toinitiate at a power setting of 801l0 and then to propagate (sometimesacross the entire specimen). However, these cracks were very tight anddid not significantly degrade the strength of the specimen. Apparentlythe Ta wires bridging the cracks act in effect as tiny reinforcing bars.With the thermal shock instrumentation used, pure refractory metalcarbides require power settings of 60 to 80 for crack initiation. Insuch carbides, however, only a slight increase in the power settingleads to rapid fracture.

It was only at power settings in excess of 200 that these materialscould be fractured. In fact, the withgrain specimens could not befractured at all, even though, at these power settings, the specimenshad so many cracks that they resembled cracked safety glass. Severalspecimens were finally forced to part into two pieces by melting the Tawires holding the cracked segments together.

The thermal shock data for these materials are summarized in Table I. Ascan be seen, two thermal shock indices are given; one pertains to crackinitiation and one pertains to fracture. This latter index is moreimportant in terms of the ultimate capability of these materials.Included in Table I for comparison are the results for with-grain RVCgraphite. As can be seen, crack initiation was not obtained with RVC;rather fractures through the washer occur.

Room temperature comparative thermal conductivity data were alsoobtained on these materials. Table II presents a comparison between theexperimental and predicted values of the thermal conductivity. The modelassumed is a dispersion of Ta wires in a continuous matrix of TaC. Theaxes of the wires are assumed to be parallel to the radial plane of thepressing, but randomly oriented in that plane. The porosityrelationships were those derived for hot-pressed ZrC.

Table I THERMAL SHOCK DATA FOR Ta WIRE REINFORCED TaC COMPOSITES ThermalShock Index Grain For Crack Material Orientation Initiation For FractureRVC Graphite 252/255 60 vol Ta wire 40 vol TaC WG 80/100 can't fracture60 vol Ta wire 40 vol TaC AG 70/80 225 70 vol Ta wire 30 vol TaC WG80/!00 cant fracture* 70 vol Ta wire 30 vol TaC AG 60/80 200 3 *Meltingof Ta fibers occurs before fracture Table II COMPARISON OF PREDICTED ANDEXPERIMETAL VALUES OF THERMAL CONDUCTIVITY OF TaC-Ta (WIRE) HOT-PRESSEDCOMPOSITE BODIES (W/cmK at Table III shows x-ray diffraction data forvarious TaC-chopped Ta wire composites. The numbers refer to the volumepercent TaC or Ta. As pressed means the composite prepared by hotpressing at l,800 C for about one-half hour. Heat treated means exposedto 2,300 C for 10 hours in a hydrogen environment after being hotpressed. It can be seen that the hot pressed composites show some slightreaction of the Ta fibers with the surrounding TaC matrix to produce thelesser carbide Ta C. The heat treated specimens having higher Ta fibercontents show a much more pronounced reaction between the fibers and thesurrounding TaC matrix to produce Ta C. The strong presence of thehydride phase indicates that even after l0 hours at 2,300 C asubstantial amount of the Ta remains noncarbided. It is not critical tothis invention that the fibers remain substantially present as themetal. The lesser metal carbides have physical and mechanical propertiesthat are sufiiciently different than those of the monocarbide matrixthat crack propagation is stopped or greatly impeded on reaching them.They thus serve to prevent or greatly inhibit fracturing insubstantially the same manner that the metal fibers do.

TABLE III X-RAY DIFFRACTION DATA FOR TaC-CHOPPED Ta WIRE COMPOSITESPhases Present Sample Description Strong Medium Weak 90 TaC-l0 Ta aspressed TaC Ta C TaC-3O Ta as pressed TaC Ta C 50 TaC-50 Ta as pressedTaC Ta,C 40 TaC-60 Ta as pressed TaC Ta C 30 TaC-70 Ta as pressed TaCTa,C TaC-l0 Ta heat-treated TaC 70 TaC-30 Ta heat-treated TaC Ta C 50TaC-50 Ta heat-treated Ta C TaH TaC 4O TaC-60 Ta heat-treated Ta,C

TaC

o.1\ 30 TaC-7O Ta heat-treated Ta C TaC TaH

What we claim is:

I. A structural material having an improved thermal stress resistance attemperatures in excess of 2,000 C comprising a composite of 50 volumepercent TaC and 50 volume percent chopped Ta fibers.

